The present invention relates generally to demodulators and more particularly to a demodulator for measurement receivers.
Measurement receivers, such as spectrum analyzers, vector signal analyzers, and the like, receive an RF input signal and down convert the RF signal to an intermediate frequency (IF) signal in a superheterodyne process to allow measurements to be made on the signal. The RF carrier signal being measured generally contain modulation information which is measured by the measurement receiver. Some measurement receiver, such as the 2715 Spectrum Analyzer, manufactured by Tektronix, Inc., include specialized circuitry for making measurements on particular types of signals. The 2715 Spectrum Analyzer includes circuitry for making automated cable television measurements on NTSC signals, such as in-service measurements for in-channel carrier-to-noise ratio, composite triple beats, and the like.
Receivers designed for receiving broadcast signal, such as radio and television signals, include front end down converter circuitry and a demodulator for extracting the modulated information from the broadcast signal. Such demodulators are designed to remove or suppress signal distortions, such as noise and the like, that are produced during the generation, transmission, and reception of the broadcast. However, signal distortions affect the quality of the broadcast signal and measuring these distortions are of interest to broadcast engineers. This is especially true for the new digital television standard adapted by the Federal Communications Commission.
The Digital Television Standard was developed by the Advanced Television Systems Committee (ATSC) to transmit high quality video, audio and ancillary data over a 6 MHZ channel. The Standard describes the channel coding and modulation RF/transmission subsystems for terrestrial and cable applications. The modulation subsystem uses a digital data stream to modulate the transmitted signal and may be implemented in two modes: a terrestrial broadcast mode (8-VSB) delivering about 19 Mbps, and a higher data rate mode (16-VSB) delivering about 38 Mbps for cable television systems where higher signal to noise is ensured.
The modulation technique implemented in the Digital Television Standard was developed by Zenith Corp. and employs vestigial sideband modulation. The overall system response of the transmitter and receiver filtering corresponds to a raised cosine filter to avoid system generated intersymbol interference. The system response is implemented with serially coupled, nominally identical root raised cosine filters in the transmitter and in the receiver.
The incoming digital data stream is randomized, forward-error-correction (FEC) encoded and interleaved. The randomized, FEC coded and interleaved data is trellis encoded as an 8-level (3-bit) one dimensional constellation. The outputs of the trellis coder are mapped into symbols that are one of eight symmetric odd-valued integer levels from xe2x88x927 to +7 units. To aid synchronization in low signal to noise and/or high multipath situations, segment and field syncs are inserted in the 10.76 Msymbols/sec symbol stream. A small pilot tone is added as well at the carrier frequency generated by offsetting the real or I channel of the complex signal containing the data and the sync pulses by 1.25 units. The offset causes the pilot tone to be in-phase with the I channel signal component. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. Alternately, the composite signal may be used to directly modulate the RF carrier.
The Hewlett-Packard HP 89441A Vector Signal Analyzer is a general purpose measurement instrument having specialized filters and processes for making measurements on a number of RF modulated signals, such as 8-VSB signals, IS-95 wireless communication signals and the like. The HP 89441A includes a superheterodyne receiver having a first LO and mixer for up-converting the incoming signal to a first IF frequency. Second and third LOs and mixers respectively generate second and third IF frequencies of 40 MHz and 10 MHz. The 10 MHz IF is digitized by an analog-to-digital converter with the digitized data being down converted to baseband data. The baseband data values are passed to a digital signal processor for FFT conversion and additional signal processing. The 89441A includes a user interface for selecting the appropriate filter for the signal being measured including no filter at all. For an 8-VSB signal, a root-raise cosine transmission system receiver filter is applied to the 8-VSB signal. One of the measurements displayed by the 89441A is a constellation display of the 8-VSB symbols in I and Q space. It has been observed that removing the root-raised cosine filter before the 8-VSB constellation measurement results in no constellation output display. From this observation and the fact that the instrument does not provide nonlinearity measurement outputs, is it assumed that only filtered 8-VSB measurements can be made with this instrument.
The received signal in transmission systems, such as the 8-VSB digital television system, generally contain distortions caused by the system""s transmitter and receiver and the medium which through the signal travels. The existence of these distortions can severely degrade the signal quality of a digitally transmitted signal. Often the distortions present in the received signal are a mixture of linear and nonlinear magnitude errors, linear and nonlinear phase error, additive noise, and phase noise. To monitor the quality of the transmitted signal and to trouble-shoot a degraded transmission system, accurate measurements of these distortions are very useful. However, traditional demodulators suppress some of these distortions, in part, by applying the transmission system receiver filter to the incoming signal. Since linear and nonlinear transfer functions are not usually commutative, meaning that the nonlinear functions observed from the demodulated baseband signal are different from the original nonlinear functions produced by the power amplifier at the transmitter, it is difficult to derive the original nonlinear transfer function from what is observed from the demodulated baseband signal for performing, for example, transmitter nonlinearity measurements, especially with a randomized digital signal. Also, strong nonlinearity causes signal spectrum spreading. The transmission system""s receiver filter could significantly attenuate the out-of-band portion of the spread spectrum signal with the loss of spectral information characterized by the nonlinear distortions.
What is needed is a demodulator for a measurement receiver that measures various type of distortions generated in a digital transmission system. Such a demodulator needs to produce multiple types of outputs for measuring linear and nonlinear type of distortions, as well as carrier phase jitter. The demodulator needs to be efficiently implemented and flexible in design for easy modification for demodulating different types of signal.
Accordingly, an object of the present invention is a demodulator that produces filtered and unfiltered signal samples where the filtered signal samples are processed through a transmission system receiver filter and the unfiltered signal samples are not.
An additional object of the present invention is a demodulator that produces synchronization parameters, a scaling factor and filter coefficients using transmission receiver filter signal samples in a first processing channel for use in producing unfiltered signal samples in a second processing channel.
A measurement receiver receives a radio frequency signal modulated with digital symbols at a symbol rate and generated by a transmission system having a transmitter filter and a receiver filter. The receiver down converts the modulated radio frequency signal to a modulated intermediate frequency signal and digitizes the signal using hardware front end circuitry to produce signal samples of the intermediate frequency signal. The measurement receiver demodulator receives the signal samples and produces filtered signal samples using the transmission system receiver filter in a first processing channel and produces unfiltered signals by bypassing the transmission system receiver filter in a second processing channel. The first processing channel further includes subchannels with one subchannel producing filtered equalized signal samples and a second subchannel producing filtered unequalized signal samples. The second processing channel further includes subchannels with one subchannel producing unfiltered equalized signal samples and a second subchannel producing unfiltered unequalized signal samples.
The first and second subchannels of the first processing channel include a down converter receiving the intermediate frequency signal samples and produces baseband signal samples. The baseband signal samples are processed by a digital filter having combined filter coefficients producing a transmission system receiver filter response and compensating for the hardware front end circuitry to generate filtered baseband signal samples. Alternatively, a first digital filter having filter coefficients compensating for the hardware front end circuitry receives the baseband signal samples and generating compensated baseband signal samples and a second digital filter having filter coefficients producing a transmission system receiver filter response receives the compensated baseband signal samples and generates filtered baseband signal samples.
A symbol timing synchronizer receives the filtered baseband signal samples, the combined filter coefficients and the baseband signal samples. A timing estimator in the synchronizer receives the filter coefficients and the baseband signal samples and generates timing phase and rate offsets parameters that are applied to a resampler in the synchronizer to produce time-aligned, filtered baseband signal samples. A scalar receives the time-aligned, filtered baseband signal samples and estimates a scaling factor that is applied to the signal samples to produce scaled, time-aligned, filtered baseband signal samples. If the modulated RF signal to the receiver contains a pilot tone, a pilot level remover may be provided to receive the scaled, time-aligned, filtered baseband signal samples that estimates and removes the pilot level from the filtered, time-aligned, scaled signal samples. The filtered, time-aligned, scaled signal samples are output as filtered unequalized signal samples from the second subchannel. Alternately, the scalar and pilot level remover may be combined. An equalizer receives the scaled, time-aligned, filtered baseband signal samples and generates equalizer coefficients that are applied to an equalization filter to produce equalized, scaled, time-aligned, filtered baseband signal samples that are output as filtered equalized signal samples from the first subchannel.
In a further embodiment of the present invention where the intermediate frequency signal samples are acquired at N times the symbol frequency, where N is greater than 1, the first and second subchannels may each include a down sampler, having a decimation factor up to N, receiving respective scaled, time-aligned, filtered baseband signal samples and the equalized, scaled, time-aligned, filtered baseband signal samples for generating down sampled scaled, time-aligned, filtered baseband signal samples and down sampled equalized, scaled, time-aligned, filtered baseband signal samples that are respectively output filtered unequalized signal samples and filtered equalized signal samples from the first subchannel.
The second processing channel receives baseband signal samples and includes a first digital filter receiving the filter coefficients that compensate for the hardware front end circuitry from the first processing channel. The filter coefficients and the baseband signal samples are applied to the digital filter to generate compensated baseband signal samples. A resampling filter receives the timing phase and rate offsets parameters from the symbol timing synchronizer of the first processing channel and the compensated baseband signal samples. The signal samples and parameters are applied to the resampling filter to produce time-aligned, compensated baseband signal samples. A scalar receives the scaling factor from the first processing channel scalar and the time-aligned, compensated baseband signal samples to generate scaled, time-aligned, compensated baseband signal samples that are output as unfiltered unequalized signal samples from the second subchannel. A second digital filter receives the equalizer coefficients from the first processing channel equalizer and the scaled, time-aligned, compensated baseband signal samples to produce equalized, scaled, time-aligned baseband signal samples that are output as unfiltered equalized signal samples from the first subchannel.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawings.